The use of a common magnetic structure which is shared by two sets of stator windings was first introduced in the late 1920s as a way to increase the power capability of large synchronous generators. See, P. L. Alger, et al., "Double Windings for Turbine Alternators, " AIEE Transactions, Vol. 49, January, 1930, pp. 226-244. Since that time, dual stator machines have been used in many other applications. These include synchronous machines with AC and DC outputs. P. W. Franklin, "A Theoretical Study of the Three-Phase Salient Pole-Type Generator with Simultaneous AC and Bridge Rectified DC Output, " IEEE Transactions on Power App. and Systems, Vol. PAS-92, No. 2, March/April 1973, pp. 543-557. Dual stator machines have also been used as current source inverters to large pumps, compressors and rolling mills driven by induction machines. T. Kataoka, et al., "Dynamic Control of a Current-Source Inverter/Double-Wound Synchronous Machine System for AC Power Supply, " IEEE Transactions on Industry Applications, Vol. IA-17, No. , 3May/June 1981, pp. 314-320. Another purpose for the use of dual stators has been to improve reliability at the system level. See, e.g., J. R. Fu, et al., "Disturbance Free Operation of a Multiphase Current Regulated Motor Drive with an Open Phase, " IEEE Transactions on Industry Applications, Vol. 30, No. 5, September/October 1994, pp. 1267-1274; J. C. Sahnon, et al., "A Split-Wound Induction Motor Design to Improve the Reliability of PWM Inverter Drives, " IEEE Transactions on Industry Applications, Vol. IA-26, No. 1, January/February 1990, pp. 143-150.
Dual stator machines are normally constructed by "splitting " the stator winding into two displaced but identical windings. See, e.g., E. F. Fuchs, et al., "Analysis of an Alternator with Two Displaced Stator Windings, " IEEE Transactions on Power App. and Systems, Vol. PAS-93, No. 6, November/December 1974, pp. 1776-1786. However, splitting the stator winding in this manner results in mutual coupling between the stators, causing circulating harmonic currents. K. Gopakumar, et al., "Split-Phase Induction Motor Operation from PWM Voltage Source Inverter, " IEEE Transactions on Industry Applications, Vol. 29, No. 5, September/October 1993, pp. 927-932. Such split stator winding machines have thus had a major drawback because the circulating currents add extra stator losses and demand larger semiconductor device ratings. In addition, there is coupling between the electromagnetic torques produced by each stator winding. See, T. A. Lipo, "A d-q Model for Six Phase Induction Machines, " International Conference on Electric Machines, Athens, Greece, 1980, pp. 860-867.